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Introduction

Igneous activity across the Gawler Craton during the Mesoproterozoic (c. 1595–1575 Ma) resulted in intense hydrothermal activity and formation of iron oxide – copper–gold (IOCG) deposits along the eastern margin of the craton. Other mineralisation has been linked to this event, including epithermal-style precious metal, lead–zinc, tin and copper occurrences along the southern margin of the Gawler Range Volcanics, on northern Eyre Peninsula. The timing and relationship of these apparently disparate mineral occurrences, however, remain poorly constrained. The dating of alunite from an area of advanced argillic alteration at Nankivel Hill provides new data on the timing of hydrothermal fluid activity that links with the c. 1590 Ma igneous event. Full details are provided in Nicolson et al. (2017). The result confirms the preservation of a Mesoproterozoic epithermal mineral system and adds support for exploration models that anticipate remnants of extensive, interrelated and varied epithermal mineralisation associated with the southern extent of the Gawler Range Volcanics.

Background

During mineral exploration by Aberfoyle Resources in 1991, siliceous breccia in low hills south of Nankivel Dam and 4.5 km WNW of Peterlumbo Hill, northern Eyre Peninsula, was recognised as evidence of shallow crustal hydrothermal activity, most probably related to igneous activity during eruption of the Gawler Range Volcanics (Coutts et al. 1991). The observation raised the potential for epithermal-style gold mineralisation. Initial surface geochemistry identified anomalous concentrations of As±Au–Ag–Pb–Cu on the northern margin of brecciated and silicified carbonate metasedimentary rocks, correlated with Katunga Dolomite of Paleoproterozoic Hutchison Group (Coutts et al. 1991). The anomalous area was referred to as the Nankivel Dam prospect and the main outcrop of siliceous breccia was later informally named Nankivel Hill (Figs 1, 2). Four short drill traverses comprising 27 shallow RAB holes (average 22 m depth, total 573 m) failed to extend the geochemical anomaly. The significance, however, for gold exploration in a previously unrecognised hydrothermal mineralising system with possible links to extrusion of the Gawler Range Volcanics was clearly established. A subvolcanic epithermal mineralisation model was developed by Aberfoyle geologists as a focus for exploration on northern Eyre Peninsula and the southern margin of the Gawler Range Volcanics. Subsequent soil sampling identified areas with anomalous silver concentration further to the NW, which were later followed up by Investigator Resources Limited and resulted in discovery, in 2011, of the Paris silver deposit, 4 km NW of Nankivel Hill (Investigator Resources 2011a, b). Total mineral resource at the Paris deposit is estimated at 9.3 Mt at 139 g/t Ag and 0.6% Pb for 42 Moz contained silver and 55 kt contained lead at a cutoff of 50 g/t Ag (Investigator Resources 2017a).

The epithermal characteristics of the Nankivel breccia were confirmed by petrographic analysis that identified alunite and dickite associated with silicification in strongly altered rock samples (Purvis and Pontifex 1991). In 1996 alteration mapping using a portable infrared mineral analyser delineated zones containing alunite, dickite and pyrophyllite (Gerakiteys 1996); the mineral assemblage is consistent with that found in advanced-argillic alteration systems and commonly associated with high-sulfidation epithermal As–Au–Cu mineralisation (e.g. Corbett and Leach 1998). In joint venture with Mount Isa Mines Ltd, three RC percussion holes (averaging 115 m) were drilled in September 1996 on Aberfoyle Exploration Licence 1841 on the flanks of the hills, close to the margins of blocky siliceous breccia outcrop. Drill cutting samples (2 m interval) were analysed for Au, Cu, Pb, Zn and As, but the results were not deemed significant at the time (Gerakiteys 1996). In the context of an advanced-argillic alteration system that may host high-sulfidation-style epithermal mineralisation however, these results and in particular the coherent anomalous arsenic results are now considered to be significant as an indicator to possible sites of nearby concealed mineralisation (e.g. Sillitoe 2015). Investigator Resources has continued to refine a model for associated Cu–Au porphyry mineralisation at depth. In September 2016, a 600 m deep drillhole intersected altered porphyritic monzonite with narrow intervals containing up to 0.16% Cu and 0.47 g/t Au; four 300 m inclined holes are proposed to be drilled during 2017 to test their refined target, in an area 1 to 2 km NW of Nankivel Hill (Investigator Resources 2017b).

Evidence of hydrothermal alteration and epithermal-style mineralisation in crystalline basement rocks along the southern margin of the Gawler Range Volcanics is now recognised as regionally extensive (e.g. Wade McAvaney and Gordon 2014). At some sites this can be demonstrated to extend into the lower units of the Gawler Range Volcanics (e.g. at Paris and Menninnie Dam). The timing of alteration and mineralisation has been widely linked to magmatic-hydrothermal processes during the c. 1590 Ma volcanic and plutonic event that formed the silicic large igneous province on the Gawler Craton, but this has proved to be difficult to confirm by direct dating methods. Epithermal mineralisation of Proterozoic age is rarely preserved in the geological record as these shallow systems are easily eroded. Also, the timing of alteration and mineralisation is usually complicated by overprinting and recrystallisation during subsequent fluid and thermal events and by the lack of dateable minerals that formed during the primary phase of mineralisation. Evidence in support of the model is significant, however, as confirmation of both the rare preservation of an ancient epithermal system and the regional prospectivity for this style of mineralisation. Investigations to determine the age of alteration at Nankivel Hill were initiated following preliminary petrology and scanning electron microscopy (Fig. 3) that reconfirmed the alunite was a primary alteration phase and probably suitable for dating by 40Ar/39Ar isotope analysis (Nicolson et al. 2017).

Alunite (KAl3(SO4)2(OH)6) within the alteration system at Nankivel Hill is well developed and coarse grained. Two samples of alunite in siliceous breccia were selected for 40Ar/39Ar dating (SA Geodata rock samples 2132172 and 2132175). The expected Proterozoic age for the alteration system raised possible complications with interpretation of the results of 40Ar/39Ar analyses and required some understanding of the regional thermal history. 40Ar is the product of the decay of 40K and is able to diffuse from a mineral lattice over geological timeframes provided the mineral is above a ‘closure temperature’ for diffusion. In reality, the ‘closure temperature’ of a mineral to Ar diffusion is more likely a temperature range, however, this range depends upon factors such as the rate of cooling, the grain size and also upon the degree of crystallinity of the mineral (e.g. McDougall and Harrison 1999). 40Ar/39Ar mineral ages can be variously interpreted as the age of mineral crystallisation, the age of cooling through a nominal closure temperature, or the age of mineral alteration. Choosing between these alternative interpretations, particularly for results from fluid alteration systems, can be assisted by comparison of results with 40Ar/39Ar dating of samples from outside the alteration system. Consequently, muscovite from samples of high-grade metamorphic rocks of Warrow Quartzite both close to and distal to the alteration system were also analysed. Muscovite has a nominal closure temperature of ~350 °C (McDougall and Harrison 1999). In addition, porphyritic dacite of the Gawler Range Volcanics was also sampled and the K-feldspar phenocrysts dated using the 40Ar/39Ar technique. K-feldspar is generally considered to have a lower closure temperature window than muscovite (~250–150 °C). A summary of the samples and the minerals that were dated is given in Table 1. Full descriptions of the samples are provided in Nicolson et al. (2017).

Table 1 Summary of samples and minerals dated by 40Ar/39Ar analysis

SA Geodata sample number

Rock type

Mineral dated

Stratigraphic unit

Easting

Northing

Age (Ma)

Age type

Comments

2079444

Schist

Muscovite

Warrow Quartzite

603397

6382850

1681 ± 5 and 1679 ± 5

Cooling age

Post-metamorphic cooling

2009371050*

Schist

Muscovite

Warrow Quartzite

581045

6381752

1674 ± 6

Cooling age

Post-metamorphic cooling

2132172

Siliceous breccia

Alunite

Hutchison Group?

599406

6384806

1586 ± 8

Hydrothermal crystallisation

Slightly disturbed age spectra. Minimum age for alunite crystallisation given.

2132175

Siliceous breccia

Alunite

Hutchison Group?

599406

6384806

c. 1590

Hydrothermal crystallisation

Slightly disturbed age spectra. Minimum age for alunite crystallisation given.

2132076

Porphyritic dacite

K-feldspar

Gawler Range Volcanics

604787

6380851

c. 900–700

Timing of alteration / recrystallisation?

Age significance uncertain

* Geoscience Australia sample

Analyses and results

All samples were sent to the Argon Geochronology Laboratory, University of Queensland, for analysis. Details of sample preparation, analytical procedures and full analytical data are given in Nicolson et al. (2017). Each sample was heated incrementally with a continuous-wave Ar-ion laser with a 2 mm wide defocused beam. The fraction of gas released was cleaned and analysed for Ar isotopes. The age of crystallisation was interpreted from incremental heating plateau ages, defined as a sequence of two or more steps corresponding to a least 50% of the total 39Ar released.

Discussion

Geological observations suggest that the regional host rocks to epithermal alteration at Nankivel Hill are metasedimentary rocks of the Hutchison Group. The 40Ar/39Ar data (Table 1) implies that these rocks cooled to temperatures below ~350 °C shortly after the Kimban Orogeny (1730–1690 Ma), as revealed by the muscovite cooling ages of c. 1680 Ma from muscovite schist samples 2079444 (SA Geodata) and 2009371050 (Geoscience Australia). Subsequently, these rocks were subjected to high-sulfidation style epithermal alteration. The timing of this alteration is suggested to be c. 1585 Ma, based on the dating of alunite within the advanced argillic alteration assemblage. However, the alunite samples do not provide flat age spectra (Fig. 4). Rather, in three-step heating experiments on three alunite grains from two different samples, the age spectra obtained show a progressive increase in age from minimum ages of c. 1400 Ma to maximum ages of c. 1600 Ma. These age spectra suggest that the alunite has not remained completely closed to Ar diffusion since initial crystallisation.

In general, alunite has been shown to retain Ar over geological time where the crystals have remained unaffected by secondary alteration processes or thermal events exceeding temperatures of ~200–220 °C (Landis, Snee and Juliani 2005). Argon is present in different sites within alunite: (i) fluid inclusions; (ii) at OH sites within the crystal structure; and (iii) in sulfate sites within the crystal structure (Landis, Snee and Juliani 2005). These three sites have different thermal Ar retentivity characteristics, such that the fluid-inclusion-held Ar is released during step heating at lower temperatures, the OH site-bound Ar is released at moderate temperatures and finally the domains within the sulfate crystal itself are the most retentive, outgassing at furnace or laser temperatures >500 °C (Landis, Snee and Juliani 2005).

Examples of 40Ar/39Ar dating of alunite from Phanerozoic (Cenozoic) ore deposits and alteration systems have demonstrated that the ages obtained from alunite in this way are geologically significant and that alunite represents a very useful mineral for dating acidic alteration episodes (e.g. Bendezú et al. 2008; Schütte et al. 2012). However, in Cenozoic examples, alunite 40Ar/39Ar age spectra typically yield relatively simple age profiles consistent with minimal post-crystallisation disturbance.

In contrast, the alunite from samples 2132172 and 2132175 show a monotonic increase in age with increase in step heating temperature. This increase in age suggests that either the alunite underwent slow cooling post-crystallisation, or that it underwent a thermal event that partially modified the age profile within the crystal. It is reasonable to assume, however, that the oldest ages represent minimum ages for the time of alunite crystallisation and hence the acid sulfate alteration event. The best estimate for the timing of this event is 1586 ± 8 Ma. This age broadly corresponds to the timing of emplacement of Hiltaba Suite granites and eruption of the Gawler Range Volcanics within the Gawler Craton, and implicates this thermal event as a driving force for the hydrothermal alteration observed at Nankivel Hill.

Conclusion

Hydrothermal alunite in the Nankivel Hill area is associated with silicification and advanced-argillic alteration. 40Ar/39Ar dating of hydrothermal alunite suggests that this alteration system is likely to have formed at c. 1590 Ma, although the slightly disturbed age spectra also suggest the alunite has experienced Ar loss during some younger thermal or fluid event(s). Metamorphic muscovite from Paleoproterozoic basement rocks (Warrow Quartzite) in the Nankivel Hill area yields 40Ar/39Ar plateau ages c. 1675 Ma, consistent with regional cooling post Kimban Orogeny below ~350 °C. The regional cooling profile is consistent with the presence of early Mesoproterozoic upper crustal rocks (i.e. Gawler Range Volcanics) in the region, and this together with the c. 1590 Ma ages for the hydrothermal alunite is consistent with the presence of upper crustal alteration systems that were broadly related to the thermal event that produced the Gawler Range Volcanics/Hiltaba Suite magmatism.

Acknowledgements

John Anderson and the team at Investigator Resources are acknowledged for their encouragement and cooperation with investigations, including sharing their ideas on the geology and mineralisation and assistance with access to samples and key field sites. Stuart McClure (University of South Australia) assisted with scanning electron microscopy and David Thiede (University of Queensland) assisted with 40Ar/39Ar analyses.